Low resistance vertical cavity light source with PNPN blocking

ABSTRACT

A semiconductor vertical light source includes upper and lower mirrors with an active region in between, an inner mode confinement region, and an outer current blocking region that includes a common epitaxial layer including an epitaxially regrown interface between the active region and upper mirror. A conducting channel including acceptors is in the inner mode confinement region. The current blocking region includes a first impurity doped region with donors between the epitaxially regrown interface and active region, and a second impurity doped region with acceptors between the first doped region and lower mirror. The outer current blocking region provides a PNPN current blocking region that includes the upper mirror or a p-type layer, first doped region, second doped region, and lower mirror or an n-type layer. The first and second impurity doped region force current flow into the conducting channel during normal operation of the light source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/014,305 entitled “LOW RESISTANCE VERTICAL CAVITY LIGHT SOURCE WITHPNPN BLOCKING,” filed Jun. 21, 2018, which is a continuation of U.S.patent application Ser. No. 15/648,260 entitled “LOW RESISTANCE VERTICALCAVITY LIGHT SOURCE WITH PNPN BLOCKING,” filed Jul. 12, 2017, now U.S.Pat. No. 10,033,156, which claims the benefit of U.S. Provisional PatentApplication No. 62/361,531 entitled “VERTICAL CAVITY DEVICES,” filed onJul. 13, 2016, and U.S. Provisional Patent Application No. 62/467,514entitled “LOW RESISTANCE VERTICAL CAVITY LIGHT SOURCE” filed Mar. 6,2017, that are all incorporated by reference herein in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Phase III SBIRContract No. W911NF-14-C-0088 awarded by the Department of Defense (DOD)funding agency, the Army Research Laboratory. The U.S. Government hascertain rights in this invention.

FIELD

Disclosed embodiments relate to semiconductor vertical cavity diodelight sources that include epitaxial heterostructures that include acavity spacer, which provide transverse optical and electricalconfinement in the optical cavities.

BACKGROUND

Vertical-cavity surface-emitting devices have generally used an oxideaperture to funnel current into small active volumes of their opticalcavities. The oxide aperture has been successfully used to create anaperture close to the device's active region and produce high efficiencythrough both mode and current confinement. However oxide vertical-cavitysurface-emitting lasers (VCSELs) are limited by internal strain, heatflow, and manufacturing non-uniformity in the oxide aperture size thatresults from the oxide formation.

RCLED's are described in U.S. Pat. No. 5,226,053. A RCLED is a lightemitting diode (LED) that generates mainly spontaneous emission andgenerally operates without a distinct threshold. Resonant cavity lightemitting diodes (RCLEDs) also use oxide apertures and operate in thespontaneous emission regime. Oxide-aperture RCLEDs suffer similarproblems to oxide-aperture VCSELs due to high internal strain,self-heating, and manufacturing non-uniformity.

While the oxide aperture has been successfully used in many VCSELdevices, it has drawbacks due to its material differences between theoxide material used to form the aperture, and the surroundingsemiconductor material. The oxide generally has a different thermalexpansion coefficient than the oxide, and proceeds through a timeddiffusion process that results in aperture size variation across aprocessed VCSEL wafer. The oxide aperture is also limited in where itcan be placed in the vertical cavity to avoid or minimize materialdegradation and strain inherent in the oxide.

Other techniques such as buried tunnel junctions or proton implantedresistive regions have also been used to obtain current and modeconfinement without the need for an oxide aperture. However the tunneljunction can lead to increased resistance and voltage drop at the highcurrent density used for the vertical cavity light source. The protonimplanted VCSEL suffers from the thick implanted region required toachieve electrical isolation, which also increases the electricalresistance. Poor optical mode behavior also results due to self-heatingand minimal built-in optical guide for the lasing mode.

Therefore the technology of vertical cavity surface emitting devices hasa remaining need for device that can provide epitaxial mode confinement,while being able to engineer the mode confinement, electrical injection,surface step height, and material quality in producing the device.

SUMMARY

This Summary is provided to introduce a brief selection of disclosedconcepts in a simplified form that are further described below in theDetailed Description including the drawings provided. This Summary isnot intended to limit the claimed subject matter's scope.

Disclosed embodiments include vertical resonant cavity light sources,such as a VCSEL, RCLED, or a surface-emitting LED that includes impurityregions placed in its cavity spacer to control the electricalconductivity in and around an injection region to the light emissionregion of the device. A feature of disclosed structures is that theelectrical conductivity is controlled by epitaxial confinementstructures to provide very low electrical resistance under normal deviceoperation by forming a PNPN blocking region in an outer current blockingregion with a conducting channel in an inner mode confinement region.The light sources can use mirrors based on distributed Bragg reflectors(DBRs) that include epitaxial DBR heterostructure mirror layers and caninclude heterostructure cavity spacers. The upper mirror (or p-typelayer above the active region) and lower mirror (or n-type layer belowthe active region) are combined with a p-doped region and n-doped regionthat all together form the PNPN current blocking region in the outercurrent blocking region to increase the current blocking while producinga low resistance current path to the active region in the inner modeconfining region. The first impurity doped region and second impuritydoped region force current flow into the conducting channel duringnormal operation of the light source.

As used herein a “cavity spacer” is defined to be a cavity region thatincludes the active region where the field undergoes phase change tocreate the resonance condition of the vertical cavity. Typically thecavity spacer thickness will be approximately an integer number ofhalf-wavelengths thickness. The cavity spacer generally may have anupper cavity spacer region and/or a lower cavity spacer region. Forexample, in a full wave cavity spacer the first mirror layer may be anAlGaAs layer that has an Al composition that is increased over thematerial of the cavity spacer. If the active region is placed near thecenter of the full-wave cavity spacer it will have an upper cavityspacer region and a lower cavity spacer region. In a half-wave cavity,in contrast, the first mirror layer may be an AlGaAs composition that islower in Al composition than the cavity spacer. It is also possible thatan active region is placed at the upper or lower edge of a cavityspacer, if properly designed to be close to a field intensity peakformed by the cavity spacer. Other materials systems using to fabricatevertical-cavity light sources such as AlGaN and GaN, or InAlGaAs withvarying Al compositions, or non-Al bearing materials such as InGaAsP,etc., may also be used in similar fashions to those disclosed below.

Layers are disclosed below that may represent epitaxial materialcompositional layers of a given material, or collections of epitaxialmaterial compositional layers grouped to perform one or more functions.For example, a DBR mirror layer is an epitaxial layer that willgenerally include various material compositional layers for theirelectronic properties while the collection of epitaxial materialcompositional layers serve as a single DBR mirror layer. This willgenerally be true, for example, for a quarter-wave optically thickmirror layer. The same may be and generally will be true for a cavityspacer layer, a current blocking layer, an active region layer, and whatis referred to as a common epitaxial layer. A low index DBR layer asused herein refers to a DBR layer for which the weighted average withinan optical field has on average a low index relative to an adjoining DBRlayer that may on weighted average have a relatively high refractiveindex. These layers can be described respectively as relatively lowrefractive index layers or relatively high refractive index layers, orlow index and high index layers respectively. Such layers in generalwill include epitaxial layers of compositionally graded materials, suchas varying Al content for the case of AlGaAs or AlGaN. This is alsotrue, for example, of cavity spacer layers and other layers that may behigh index or low index layers.

Upper and lower and above and below are defined herein to account forwhich layer or interface is formed in regions relative to the substrateon which the light emitter is formed. Layers or interfaces referred toupper and above relative to another layer or interface are further fromthe substrate on which the light source is produced, than layers areinterfaces that are referred to as lower or below relative to a layer orinterface. The substrate in any case may ultimately be removed aftercompletion of the epitaxial growth upon further processing of theepitaxial material used to form the light source.

The conducting channel defined herein may be a p-type conducting channelor an n-type conducting channel formed in the cavity spacer byprocessing that is performed at least partly outside an epitaxial growthsystem. The epitaxial growth is at least a two-step epitaxial growthwith process steps performed at least partly outside the epitaxialgrowth system being used in-between the two epitaxial growth steps. Theconducting channel can be formed using atom diffusion, ion implantationand activation, implantation and diffusion, or epitaxial growth, orsimilar approaches of introducing impurity atoms into the semiconductorin the region desired for the conducting channel. Impurity doped regionsepitaxially grown into the cavity spacer and/or active region then formportion(s) of the current blocking region outside the conductingchannel. Heterojunctions can be used to increase the blocking voltageand decrease capacitance by increasing the thickness of depleted regionsthat may exist between impurity regions. Grown-in doping profiles withlow diffusion coefficients can meet requirements for depletion regionwidths to maintain p and n regions under forward bias for currentblocking. Combining these depletion regions with heterojunctions enablesPNPN current blocking outside the conducting channel, while maintaininghigh quality cavity spacer design for confining the laser's opticalfield.

The PNPN current blocking region includes a common epitaxial layer uponwhich device processing is performed between two epitaxial growth steps.The common epitaxial layer is defined herein as an epitaxial layer thatincludes at least one epitaxially regrown interface in the outer currentblocking region. The common epitaxial layer extends over the outer PNPNcurrent blocking region and extends over an internal mode confinementregion. The common epitaxial layer also may include at least a portionof the conducting channel within its inner mode confinement region. Theuse of the common epitaxial layer enables high crystal quality whilemodifying shallow impurity profiles in the light source. The commonepitaxial layer that exists in the outer current blocking region and inthe inner mode confinement region, with at least a regrown interface inthe outer current blocking region, enables the PNPN current blockingregion to include the cavity spacer of the vertical-cavity light source.The common epitaxial layer can include a shallow mesa within the innermode confinement region to increase the optical mode confinement of thevertical cavity light source.

The common epitaxial layer is designed to match the optical propertiesof the remaining layer of the vertical-cavity emitter. The commonepitaxial layer may adjoin a cavity spacer layer, or a portion of such alayer. However, the common epitaxial layer should be designed so as notto disrupt the optical cavity mode of the light source. In this aspectits thickness and refractive index should be designed to producenecessary resonance for efficient operation of the light source.

Low cost manufacturing is also maintained by the reliance on epitaxialmaterial properties in processing that can be deposited with highaccuracy and quality. This enables the advanced materials that includediffusion layers and sacrificial layers that can be processed with highreproducibly. Diffusions or implants can be designed with precise dopantamounts to control diffusion extent. Simple low cost annealing steps canthen be used to form the current blocking and conducting channelregions, and these can be maintained in-situ with the epitaxial growthprocesses.

Disclosed impurity profiles and heterojunctions of vertical cavity lightsources enable the conducting channel and current blocking region to beformed using p and n impurity regions that can be formed very close tothe active region, and even in the active region. The conducting channelcan extend the length of the region of highest current density, in thecavity of the light source. The conducting channel length can thus bekept short to decrease electrical resistance and operating voltage ofthe device by utilizing fabrication methods of the disclosed lightsources that maintain high epitaxial material quality in the activeregion and cavity spacer of the light source. The electrical injectionthrough the short conducting channel can reduce the electricalresistance of the light source and can increase its efficiency, whilemaintaining high reliability and low capacitance. Reliance on shallow pand n impurities that can be introduced with minimal defects enable veryhigh material quality to be maintained. Semiconductor vertical-cavitylight sources that include the conducting channel with index guides arealso disclosed.

Disclosed embodiments include a semiconductor vertical light sourceincluding an upper p-type mirror or upper p-type layer, and a lowern-type mirror or lower n-type layer. An active region is between theupper and lower mirror. The light source includes an inner modeconfinement region and outer current blocking region. The outer currentblocking region includes a common epitaxial layer that includes anepitaxially regrown interface that is between the active region andupper mirror, and a conducting channel including acceptors is in theinner mode confinement region. The current blocking region includes afirst impurity doped region with donors between the epitaxially regrowninterface and active region, and a second impurity doped region withacceptors is between the first doped region and lower mirror. The outercurrent blocking region provides a PNPN current blocking region thatincludes the upper mirror or a p-type layer above the active region,first doped region, second doped region, and lower mirror or an n-typelayer below the active region. The first impurity doped region andsecond impurity doped region force current flow into the conductingchannel during normal operation of the light source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional diagram of an examplesemiconductor vertical cavity diode light source that includes a commonepitaxial layer, and impurity doped regions of the cavity spacer.

FIG. 2 is a schematic cross sectional diagram of an expanded view ofimpurity doped regions in the cavity spacer and upper and lower DBRmirror pairs closest to the cavity spacer.

FIG. 3 is a schematic cross sectional diagram of an expanded view ofimpurity doped regions in the cavity spacer and upper and lower DBRmirror pairs closest to the cavity spacer.

FIG. 4 is a schematic cross sectional diagram of an expanded view ofimpurity doped regions in the cavity spacer and upper and lower DBRmirror pairs closest to the cavity spacer.

FIG. 5 is a schematic cross sectional diagram of impurity doped regionsand layer structure in a half-wave cavity spacer of a vertical cavitydiode light source.

FIG. 6 is a schematic cross sectional diagram of impurity doped regionsand a layer structure in a half-wave cavity spacer of a vertical cavitydiode light source.

DETAILED DESCRIPTION

Disclosed embodiments in this Disclosure are described with reference tothe attached figures, wherein like reference numerals are usedthroughout the figures to designate similar or equivalent elements. Thefigures are not drawn to scale and they are provided merely toillustrate the disclosed embodiments. Several aspects are describedbelow with reference to example applications for illustration. It shouldbe understood that numerous specific details, relationships, and methodsare set forth to provide a full understanding of the disclosedembodiments.

One having ordinary skill in the relevant art, however, will readilyrecognize that the subject matter disclosed herein can be practicedwithout one or more of the specific details or with other methods. Inother instances, well-known structures or operations are not shown indetail to avoid obscuring structures or operations that are notwell-known. This Disclosure is not limited by the illustrated orderingof acts or events, as some acts may occur in different orders and/orconcurrently with other acts or events. Furthermore, not all illustratedacts or events are required to implement a methodology in accordancewith this Disclosure.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of this Disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5.

Disclosed semiconductor light sources provide current blocking regionsor conducting channels that include cavity spacers of vertical cavitylight sources. These current blocking and conducting channel regions canbe fabricated using combinations of epitaxially grown-in and diffusedand/or implanted impurities that retain high material quality uponprocessing and epitaxial regrowth steps. Placing the current blockingregion and the conducting channel in the cavity spacer can shorten thedistance for which hole current should travel at approximately itshighest current density. In addition, the disclosed PNPN currentblocking regions formed normal to the growth plane can limit currentspreading in the cavity spacer to reduce threshold and increase slopeefficiency.

FIG. 1 shows a schematic cross sectional diagram of an examplesemiconductor vertical cavity diode light source 100 that includes anepitaxial index guide shown as an ‘index guide’ 131 with a conductingchannel 135 in the inner mode confinement region 133, and an outercurrent blocking region 132 that forces current flow through the indexguide 131 and into the conducting channel 135 under normal deviceoperation. Electrodes 121 and 111 are shown on the lower side of thesubstrate 105 and the top of the upper mirror shown as an upper DBR 110,respectively, but these electrodes can have different configurations.The substrate 105 can comprise a GaAs substrate. Alternatively, thesubstrate 105 may comprise other III-V compound semiconductors such asGaN or InP, or another substrate of suitable material for fabricating avertical cavity light source diode. The electrodes 111, 121 may beplaced within the DBR mirror layers as well, for example.

The common epitaxial layer 190 facilitates fabrication through epitaxialregrowth. The common epitaxial layer 190 is designed to match the phaseconditions needed to match the optical phase conditions of the lightsource. The upper DBR 110 includes the common epitaxial layer 190. Thecommon epitaxial layer 190 is designed meet phase conditions neededbetween the cavity spacer layer 150 and the mirror layers of the upperDBR 110. The epitaxial regrowth interface 191 is shown in FIG. 1 in theouter current blocking region 132 between the common epitaxial layer 190and the next upper adjoining layer of the upper DBR 110. The regrowninterface 191 includes and forms the index guide 131 in the inner modeconfinement region 133. By retaining the common epitaxial layer 190 alsowithin the inner mode confinement region 133, very low opticalscattering loss can be achieved in the light source 100. Thiscontributes to high efficiency of light source 100. At the same time,the light source 100 fabrication uses conducting channel 135 fabricatedin the same common epitaxial layer with the epitaxially regrowninterface 191 provided by the common epitaxial layer 190 to provideefficient electrical injection into the active region 115. The commonepitaxial layer 190 is thus included in the outer PNPN current blockingregion 195 to provide electrical isolation, and in the inner modeconfinement region that includes conducting channel 135 to provide lowelectrical resistance injection into the light source 100.

The epitaxial regrown interface 191 of the common epitaxial layer 190can generally be identified by analytical techniques such as secondaryion mask spectroscopy (SIMS) used to measure dilute impurityconcentrations, and/or transmission electron microscopy (TEM) imagingused to directly examine the structure in an image cross-section.Increased impurity content is generally collected from the fabricationperformed between the two growth steps. The TEM imaging can trace theepitaxial regrown interface of the common epitaxial layer 190 betweenthe outer current blocking region 132 and the inner mode confinementregion 133 to approximately determine the height of the step ΔL. It canalso be identified through the impurity profiles that exist in the outercurrent blocking region 132 and the inner mode confinement region 133within the PNPN blocking region 195. These impurity profiles require thecommon epitaxial layer 190 and regrowth interface 191 to properly placethem in the PNPN structure.

Optical scattering is reduced by minimizing ΔL to only a small fractionof a quarter thickness relative to the wavelength of the light source's100 light emission. For example, at a wavelength of 990 nm andconsidering a light source 100 produced on a GaAs substrate 105, andAlGaAs upper and lower DBR layers corresponds approximately to a ˜700 Å.Values of ΔL<<700 Å can thus produce low optical scattering loss withinthe inner mode confinement region 133. Yet even values of ΔL>25 Å canproduce effective mode confinement and also increase the efficiency oflight source 100. Values of ΔL≤250 Å can produce tight confinement ofthe optical mode to the inner mode confinement region 133, even forsmall confinement regions.

The PNPN current blocking region 195 including the common epitaxiallayer 190 and the regrown interface 191 includes the p-type upper DBR110, a first n-type impurity region 160 and a second p-type impurityregion 170, and the n-type lower DBR 120. The PNPN blocking region 195is formed by p-doping of upper DBR 110, n-type impurity region 160,p-type impurity region 170, and lower n-doped mirror 120. There areundoped depleted regions in-between these doped regions. The opticaldesign of the cavity 145 including the upper DBR 110 and lower DBR 120,cavity spacer layers 150 and 151, and active region 115 are combinedwith first impurity doped region 160 and second impurity doped region170 to form the PNPN current blocking region 195. In order to insuresufficient blocking for efficient device operation, the first and secondimpurity doped regions 160 and 170 are chosen with sufficient impurityconcentrations to limit total depletion widths and prevent chargetransport through the PNPN current blocking region 195 that exists inthe outer current blocking region 132, under normal device operation.

As describe above, the first and second impurity regions 160 and 170 areshown in the cavity spacer layer 150, and together with doped regions inupper DBR 110 and lower DBR 120 form the PNPN current blocking region195. Therefore the layers of the cavity spacer 150 and 151, and activeregion 115, are used for current blocking and optical mode confinement.In FIG. 1 the common epitaxial layer 190 of upper DBR 110 above thecavity spacer 150 includes no intentional doping. The adjoining DBRlayer in upper DBR 110 that also includes regrown interface 191 and whatmay be one or more subsequent DBR layers are intentionally doped withgrown in impurities (acceptors) to maintain p-type conductivity. Firstimpurity region 160 is a donor impurity to form an n-type region justbelow common epitaxial layer 190. The first impurity region 160 may beimplanted and activated, or epitaxially grown-in. It is also possiblethat the first impurity region 160 be located in the common epitaxiallayer 190, and can be implanted, epitaxially grown-in, or diffused.

The donor impurity concentration in the first impurity region 160 shouldbe sufficiently high to avoid depletion, or when depleted should retainsufficient current blocking. Second impurity region 170 is also formed,ideally by a grown-in acceptor impurity region, and placed close to theactive region 115 at the center of the cavity spacer. The secondimpurity region 170 may modulation dope the active region 115 so thatactive region 115 includes excess equilibrium hole charge. As with donordoping in the first impurity region 160, acceptor doping in the secondimpurity region 170 generally includes a sufficient acceptorconcentration to maintain p-type conductivity to achieving high blockingvoltage in the outer current blocking region 132. Lower DBR 120 is dopedwith donor impurities and completes the PNPN current blocking region195.

The formation of the first acceptor doped region that forms the upper pregion of the PNPN above the regrown interface 191 of the commonepitaxial layer 190 can create a double heterostructure barrier betweenthe first p region of the upper DBR 110 and the first n region that isthe donor doping of the first impurity region 160. The doubleheterostructure formed by the common epitaxial layer 190 can thereforesuppress hole injection into the current blocking region 195, whichincreases the blocking voltage of the PNPN blocking region 195. It isnoted that the common epitaxial layer 190 may be acceptor doped or donordoped and maintain the PNPN blocking. However, along with reducedleakage current, an undoped common epitaxial layer 190 can add to thetotal depletion thickness, reduce electrical capacitance, and ease theformation of conducting channel 135 compared to the case that the commonepitaxial layer 190 is donor doped.

Placing the p-type doped second impurity region 170 below the donordoped first impurity region 160 above and in close proximity to theactive region 115 also uses the heterostructure active region tosuppress electron injection from lower DBR 120 in to the currentblocking region 195. Suppressing both electron injection from lower DBR120 and hole injection from the upper DBR 100 can then be used toproduce a large blocking voltage and achieve efficient electricalinjection, even for small active area vertical cavity light sources.

Conducting channel 135 can be formed by either diffusion of acceptors,implantation of acceptors, or implantation and diffusion of acceptorsimpurities. Conducting channel 135 can also be formed through grown-inacceptor impurities if the common epitaxial layer 190 is also acceptordoped. The conducting channel 135 formed by introducing the acceptorsselectivity into a grown-in first impurity region 160 containing donorimpurities that may also exist in the inner mode confinement region 133counter-dopes this region in cavity spacer 150 and converts it fromn-type conductivity to p-type conductivity within the inner modeconfinement region 133 and conducting channel 135. Diffusion of columnII acceptors such as Be, Zn, or Mg especially can create this dopanttype conversion while retaining high material quality that enables thediffusion to be performed in the cavity spacer. These column IIimpurities can be epitaxially grown-in to sacrificial layers, and thenpatterned after growth to form selective acceptor diffusion sources. Oralternatively, the acceptor impurities can be implanted into a surfacelayer of the III-V epitaxial structure and subsequently annealed anddiffused. The acceptor diffusion may also be performed from an externalsource, such as a vapor source.

The column II acceptors are generally preferable since they haverelatively high diffusion coefficients at sufficiently low temperaturesfor the semiconductor crystal that can limit causing defects. The columnII acceptors diffuse through a substitutional/interstitial mechanismthat can be performed at temperatures generally between 550° C. and 700°C.

On the other hand column IV acceptors such as carbon have much lowerdiffusion coefficients, and thus remain stable in the lattice at hightemperature. Carbon may be preferable to use if the common epitaxiallayer 190 and the conducting channel 135 include grown-in acceptors inthe second impurity region 170 desired for p-type conductivity to formthe current blocking region 195. The p-type conducting channel 135 canthen be formed between subsequent epitaxial growth steps throughdiffusion and/or ion implantation of the donor impurity region 160.

FIG. 1 also shows that the index guide 131 can use a step in the crystalsurface of height ΔL between the inner mode confinement region 133 andthe outer current blocking region 132. This step height may formed byetching a surface layer to transfer the pattern into the startingepitaxial surface. For example, a high concentration of acceptorimpurities may be grown into a thin surface layer on the commonepitaxial layer 190, and then removed along with its impurities fromregions that become the current blocking region 195. It may also be ahigh concentration of acceptor impurities are implanted in a blanketcoverage, again with the implant designed to maintain the acceptorsmainly in a thin layer above the common epitaxial layer 190. This thinlayer can again be patterned and removed outside the index guide 131.

Alternatively, implanting selectively only into the region of the indexguide 131 can also be performed, with implant conditions set to minimizedamage of the crystal. This can be achieved because only very shallowimplants are needed. Thus conducting channel 135 can be formed againminimizing defects that may be created in its formation.

The second impurity region 170 is doped ideally with grown-in acceptorimpurities that can be placed close to the active region 115, with itsthickness minimized so that a depletion region easily forms between itand the first impurity region 160. First impurity region 160 may receivedonor doping of ˜10¹⁷ to ˜5×10¹⁹ cm⁻³ or greater, depending on itsthickness and surrounding regions. The conducting channel 135 need onlyextend through the donor doped first impurity region 160 to form theinjection path into the active region 115. However extending it to reacha carbon doped second impurity region 170 or even into the active region115 can reduce electrical resistance in the vertical cavity light source100.

It can also be desirable to minimize the donor impurity in the firstimpurity region 160, since the donor impurities may also exist in theconducting channel 135 if the donor impurities are grown-in. If thefirst impurity region 160 is implanted the implantation will also createdamage. Though this damage is reduced through use of low implant energy,low dose and annealing, these donor impurities may reduce hole mobilityin the conducting channel 135 within inner mode confinement region 133that overlaps the first impurity region 160. Therefore minimizing thedonor concentration while maintaining sufficient current blocking inPNPN region 195 can reduce the electrical resistance of the lightsource.

It may also be that a minimum concentration of donor impurities in thefirst impurity region 160 is needed to overcome an unintentionally highbackground of acceptor impurities. Typically, the conducting channel 135comprises acceptor impurities with concentration of ≥5×10¹⁶ cm⁻³, andmay be ≥5×10¹⁸ cm⁻³. More generally the conducting channel acceptorconcentration chosen to generate a hole charge concentration of ˜5×10¹⁷cm⁻³ to ˜2×10¹⁸ cm⁻³ through much of its extent. In this case donorimpurity region 170 doped with donors may include a donor impurityconcentration of 10¹⁷ cm⁻³ to 10¹⁹ cm⁻³. Spacing the first impuritydoped region 160 and second impurity doped region 170 as shown in FIG. 1with undoped region(s) in between also reduces the impurityconcentrations needed to form the PNPN blocking region 195.

FIG. 2 shows a second vertical-cavity light source (only partiallyshown) for which the common epitaxial layer used for epitaxial regrowth290 is now partially doped with an acceptor. An index guide 231 can beformed in the inner mode confinement region 233 as shown and as inFIG. 1. Lower DBR 220 is doped n-doped. The donor doped first impurityregion 260 is formed below the common epitaxial layer 290, and theregrown interface 291 exists between the common epitaxial layer 290 andthe acceptor doped second impurity region 270 is formed next to theactive region 215. In addition, the conducting channel 235 is formedfully extending into the cavity spacer 250 and into the acceptor dopedsecond impurity region 270. Carbon is a desirable acceptor impurity thatcan be epitaxially grown into the crystal and maintains a low diffusioncoefficient at high crystal temperature. Conducting channel 235 can bediffused from grown in acceptor impurities such as Zn, Be, or Mg, orsuch impurities can be implanted and the crystal annealed. Diffusion ingeneral will also occur when the acceptor impurities are implanted andthe crystal annealed, especially with column II acceptors that havesubstitutional-interstitial site changes that occur relatively easily inthe diffusion process.

The PNPN current blocking region (PNPN blocking region) 230 in this casecan include depleted regions due to light or no intentional impuritydoping between first impurity region 260 and second impurity region 270,and between the first impurity region 260 and the doped region in thecommon epitaxial layer 290. The lower DBR 220 that is n-doped completesthe PNPN blocking region 230 that produces current blocking and iscontained in the outer current blocking region 232. For example, adepletion width for GaAs material that uses p and n doping atconcentrations of 10¹⁸ cm⁻³ on each side in an abrupt junction has adepletion width at room temperature of ˜650 Å. Therefore abruptjunctions placed in the cavity spacer 250 can be used to produce currentblocking. The thickness of cavity spacer 250 may be ˜900 Å to ˜1500 Åfor arsenide based vertical cavity light sources, so that this dopinglevel can provide current blocking without depleting the first andsecond impurity regions 260 and 270. Higher doping levels and/or the useof heterojunctions may be used if needed to increase the currentblocking and for nitride vertical cavity light sources, for example,that operate with shorter wavelength and thus thinner cavity spacers.Lower doping levels may also be used for longer wavelengthvertical-cavity light sources or longer cavity light sources that usethicker cavity spacer layers.

For forming the conducting channel 235 it can be an advantage to keepthe first impurity region 260 that includes donor impurities thin, orwith some doping gradient. This can ease the fabrication process to formthe conducting channel 235 through the common epitaxial layer 290 andinto the cavity spacer 250. The common epitaxial layer 290 can remainpartially depleted to increase the blocking voltage by limiting holeinjection into the PNPN blocking region 230. A heavily doped upperregion of the common epitaxial layer 290 can help however to funnelcurrent into the conducting channel 235. Because the hole current shouldfunnel to its minimum in much of the conducting channel 235, its currentfunneling properties can decrease the device resistance.

Index guide 231 may be formed by selective etching of GaAs, InGaAs, andAlGaAs, for example, if the vertical cavity light source is anarsenide-based device. It could also be a nitride or phosphide device,or potentially from other semiconductor materials. For a nitride-baseddevice the selective etching can be of GaN, InGaN, or AlGaN, forexample. For a phosphide-based device (e.g., an InP substrate) thematerials may be InGaAsP, InGaAs, or InGaAlAs. Selective etching andreliance on the common epitaxial layer 290 through which the conductingchannel 235 is formed can improve fabrication and yield. Conductingchannel 235 can be formed by diffusing an impurity from a selectivelyetched crystal region that contained a high concentration of grown-inacceptor impurities just after crystal growth and had this highly dopedregion removed in the outer current blocking region 232. A shallowcrystal surface step can be formed in this case, and provide opticalmode confinement.

The conducting channel 235 may also have been formed for example from animplanted region, and this implant could have been a blanket implantcovering the crystal surface, or a selective implant due to masking. Ifthe conducting channel 235 is formed from a blanket implant, and theimplant is shallow to remain close to the crystal surface, the implantedregion can be removed by etching in the regions outside the conductingchannel and subsequently diffused. The index guide 231 with a non-zerocrystal step can be formed in this case.

Alternatively, the conducting channel 235 may be formed from a selectiveimplant of acceptor impurities only into the inner mode confinementregion 233. There may or may not be a surface step formed in this case,giving the possibility that the index step 235 may have a ΔL=0. A zeroindex guide can help to select single mode operation.

FIG. 3 shows an example partial vertical-cavity light source for whichthe conducting channel 335 is formed through a thicker n-type firstimpurity region 360. In this case the n-type doping level of the firstimpurity region 360 is thicker and may be much lower concentration than,for example, the donor doping in the first impurity region 260 in FIG.2. Common epitaxial layer 390 is also shown partially doped, which canreduce the electrical resistance of the light source. The epitaxiallyregrown interface 391 of the common epitaxial layer 390 is shown in FIG.3. Index guide 331 can be formed using the same process described forFIG. 2 to form index guide 231, but with etch conditions used to leave aplanar interface with ΔL=0. Conducting channel 335 can be formed in mostof the upper cavity spacer 350, while the lower cavity spacer 351remains unintentionally doped in a significant part.

The first impurity region 360 can be designed with appropriate n-typedoping level so that it remains at least partially undepleted, while atthe same time the donor doping is sufficiently light so as to not limitmobility of holes in the conducting channel 335. Because more of thecavity spacer 350 includes first impurity region 360, than for theembodiment of FIG. 2, the impurity doping level can be reduced. Thisdoping design can remain relatively lightly doped with donor impuritiesin first impurity region 360 and both eases the acceptor diffusion oractivation of acceptor implants used to form conducting channel 335, andto maintain low impurity scattering in the conducting channel 335relative for the acceptor doping level for the mobile hole charge. Thusthe mobile hole charge can maintain higher mobility to produce a lowerelectrical resistance channel 335. If the donor impurity used to formfirst impurity region 360 is implanted, defects can be minimized becauseof the low energy implant that can be used, and the implanted region canrequire a less extreme anneal to activate the implant.

The PNPN blocking region 330 in the outer current blocking region 332 isformed by the common epitaxial layer 390 that is part of the upperp-type DBR, n-type first impurity region 360, p-type second impurityregion 370, and the n-type lower DBR 320. Active region 315 may beundoped, though it may also include equilibrium hole charge due tomodulation doping by the second impurity region 370. If the verticalcavity light source is a VCSEL, the inner mode confinement region 333can force more preferential single mode lasing due to the index guide331 having ΔL=0.

FIG. 4 shows a partial vertical-cavity light source with furthermodifications in the design of the donor doped first impurity region 460and the acceptor doped second impurity region 470. Second impurityregion 470 is now moved into the active region 415 of the light source.The common epitaxial layer 490 is used for regrowth, and also shown withits epitaxially regrown interface 491. The PNPN blocking region 430formed in the outer current blocking region 432 is formed by theacceptor doping in the common epitaxial layer 490, the donor doping ofthe first impurity region 460, the acceptor doping of the secondimpurity region 470, and the donor doping of the lower n-type DBR 420.

Conducting channel 435 is formed as described above for conductingchannel 335 of FIG. 3, conducting channel 235 of FIG. 2, or conductingchannel 135 of FIG. 1. The common epitaxial layer 490, and index guide431 along with the electrical injection path of the conducting channel435 form the mode confinement in the inner mode confinement region 433.Lower DBR 420 can be n-doped, and cavity spacer region 451 may also ben-doped or unintentionally doped. If the cavity spacer 451 is donordoped then the PNPN blocking region 430 would be formed by the acceptordoping of the common epitaxial layer 490, the donor doping of impurityregion 460, the acceptor doping of the second impurity region 470, anddonor doping sufficient to make the lower cavity spacer 451 n-type.

As in FIG. 3, heavily doping the common epitaxial layer 490 fully withacceptors can decrease the resistance of the vertical cavity lightsource by enabling more efficient current funneling into conductingchannel 435. This current funneling is most important close to thecavity spacer so that current flowing with its highest current density(restricted to inner mode confinement region 433) has the shortestdistance to flow.

FIG. 5 shows a schematic structure of a vertical-cavity light source 500based on a half-wave cavity spacer for the laser cavity 545. In thisembodiment the common epitaxial layer 590 used for epitaxial regrowth isdesigned to provide resonance to form a half-wave cavity based on itsspacers. The common epitaxial layer 590 is thus designed to match theoptical phase conditions necessary to match the lower DBR 520, the lowercavity spacer 551, the active region 515, and the upper DBR 510. Thecommon epitaxial layer shown in the embodiment of FIG. 5 has an opticalthickness of approximately one-fourth wavelength in the material. Itsdesign thickness depends on details of the surrounding layers.

The epitaxial regrown interface 591 is shown in FIG. 5. Light source 500uses a PNPN blocking region 595 in the outer current blocking region 532including the common epitaxial layer 590. Conducting channel 535 can beformed through the common epitaxial layer 590 by acceptor diffusion,implantation, or implantation followed by diffusion of acceptorimpurities, as well as by grown in acceptor impurities.

The index guide in this case can be maintained at a zero-index value,for which ΔL˜0, or include other values of ΔL. In this case the commonepitaxial layer 590 is through which the conducting channel 535 isformed. The light source 500 is generally epitaxially grown on substrate505 and includes lower electrode 521 and upper electrode 511. Impurityregion 560 is a donor-doped region to produce n-type conductivity whilethe p-type second impurity region 570 is placed in the active region515. The PNPN blocking region 595 is formed by the upper p-type DBR 510,the n-type first impurity region 560, the p-type second impurity region570 and the lower n-type DBR 520. The use of half-wave cavity spacerscan produce a shorter photon lifetime than the full-wave cavity spacer,and a short conducting channel 535.

FIG. 6 shows a schematic structure of vertical-cavity light source 600based on a half-wave cavity spacer for cavity 645. The common epitaxiallayer is in the upper cavity spacer 690 used for epitaxial regrowth, andits regrown interface 691 is shown in FIG. 6. Light source 600 uses asubstrate 605 on which is generally epitaxially grown a lower DBR 620,cavity spacer 651, active region 615, the common epitaxial layer 690,and upper DBR 610. Electrodes 621 and 611 are formed on the epitaxialmaterials for electrical contacting. Layer 690 forms a common epitaxiallayer that includes conducting channel 635 and the donor doped firstimpurity region 660. The inner mode confinement region 633 can alsoinclude an index step an example ΔL, and in this case ΔL is assumed tobe zero and not shown.

Acceptor impurity doping in the second impurity region 670 is now formedpartially in the lower cavity spacer 651 and partially in the n-type DBR620 in its first DBR layer adjoining cavity spacer 651. Second impurityregion 670 is formed by an implantation to peak its concentration belowthe active region 615 as shown. The second impurity region 670 can beimplanted and in general extend into the lower n-type DBR 620 as shownto increase the current blocking. Although different acceptors can beconsidered, beryllium has benefits including having a relatively lightatomic mass to enable use of a reduced implant energy, and can beactivated at relatively low annealing temperature. The PNPN blockingregion 695 is formed by the upper p-type DBR 610, the donor doped firstimpurity region 660, the acceptor doped second impurity region 670, andthe n-type portion in the outer current blocking region 632 of the lowern-DBR 620.

The donor doped first impurity region 660 can be formed throughepitaxial growth, implanted, or diffused. If diffused, the diffusion cantake place in the same epitaxial growth step following formation of theimpurity regions through implantation and surface patterning in theepitaxial growth system. If donor doped first impurity region 660 isformed through epitaxial growth the conducting channel 635 may be formedby diffusion of an acceptor impurity or implantation, or implantationand diffusion.

The disclosed conducting channels and current blocking regions can beformed in half-wave cavity vertical cavity light sources, or othermultiples of half-wave cavities. Both active region placement andimpurity doping regions should be designed to account for the differentstructures. Techniques disclosed above can also form similar indexguides in half-wave or other length optical cavities.

Further embodiments can make use of the disclosed conducting channelsand current blocking regions that do not require a doped upper mirror ordoped lower mirror. These structures may be desired to reduce opticalloss or circumvent mirror layers that may otherwise cause highelectrical resistance. For example, dielectric mirrors may be used inplace of semiconductor mirrors, as long as one or more sufficientlydoped layers are included above the epitaxial regrown interface of thecommon epitaxial layer, and one or more sufficiently doped layers areincluded below the vertical cavity light source's active region. In thisway, the PNPN outer current blocking region can be formed withoutrelying on any doped mirrors. Combinations such as an n-doped lowermirror and undoped upper mirror, or doped upper mirror and undoped lowermirror may be used, while retaining the PNPN blocking.

For example, if a cavity spacer layer below an active region issufficiently n-doped, the lower DBR may be undoped or even dielectriclayers to make the DBR. Use of the common epitaxial layer with itsregrown interface, a p-doped layer above the regrown interface of thecommon epitaxial layer, and a first donor doped region and secondacceptor doped region can implement a disclosed PNPN blocking currentblocking region.

While various disclosed embodiments have been described above, it shouldbe understood that they have been presented by way of example only, andnot limitation. Numerous changes to the subject matter disclosed hereincan be made in accordance with this Disclosure without departing fromthe spirit or scope of this Disclosure. In addition, while a particularfeature may have been disclosed with respect to only one of severalimplementations, such feature may be combined with one or more otherfeatures of the other implementations as may be desired and advantageousfor any given or particular application.

Thus, the breadth and scope of the subject matter provided in thisDisclosure should not be limited by any of the above explicitlydescribed embodiments. Rather, the scope of this Disclosure should bedefined in accordance with the following claims and their equivalents.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which embodiments of the inventionbelongs. It will be further understood that terms, such as those definedin commonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand will not be interpreted in an idealized or overly formal senseunless expressly so defined herein.

The invention claimed is:
 1. A semiconductor vertical resonant cavitylight source, comprising: an upper p-type mirror and a lower n-typemirror; an active region for light generation between said upper mirrorand said lower mirror; said light source including an inner modeconfinement region and an outer current blocking region; said outercurrent blocking region comprising a common epitaxial layer thatincludes an epitaxially regrown interface extending over said inner modeconfinement region and over said outer current blocking region which isbetween said active region and said upper mirror; and a conductingchannel comprising acceptor impurities in said inner mode confinementregion; wherein said outer current blocking region provides a PNPNcurrent blocking region comprising said upper mirror; a first impuritydoped region comprising donor impurities between said epitaxiallyregrown interface and said active region; and a second impurity dopedregion comprising acceptor impurities between said first impurity dopedregion and said lower mirror; and wherein said light source isepitaxially grown on a GaN substrate.
 2. The light source of claim 1,further comprising at least one cavity spacer layer between said uppermirror and said active region.
 3. The light source of claim 2, whereinsaid conducting channel electrically contacts said cavity spacer layer.4. The light source of claim 2, wherein said conducting channel extendsinto said cavity spacer layer.
 5. The light source of claim 1, whereinsaid conducting channel extends into said active region.
 6. The lightsource of claim 1, wherein said common epitaxial layer comprises adistributed Bragg reflector (DBR) layer.
 7. The light source of claim 1,wherein said lower mirror includes an acceptor doped region.
 8. Thevertical cavity light source of claim 1, wherein said light sourcecomprises a vertical-cavity surface-emitting laser (VCSEL).
 9. The lightsource of claim 1, wherein said second impurity region is included in atleast part of said active region.
 10. A semiconductor vertical resonantcavity light source, comprising: an upper mirror and a lower mirror; anactive region for light generation between said upper mirror and saidlower mirror; said light source including an inner mode confinementregion and an outer current blocking region; said outer current blockingregion comprising a common epitaxial layer that includes an epitaxiallyregrown interface extending over said inner mode confinement region andover said outer current blocking region which is between said activeregion and said upper mirror; an upper p-type layer above saidepitaxially regrown interface; a lower n-type layer below said activeregion; and a conducting channel comprising acceptor impurities in saidinner mode confinement region; wherein said outer current blockingregion provides a PNPN current blocking region comprising said upperp-type layer; a first impurity doped region comprising donor impuritiesbetween said epitaxially regrown interface and said active region; and asecond impurity doped region comprising acceptor impurities between saidfirst impurity doped region and said lower n-type layer below saidactive region; and wherein said light source is epitaxially grown on aGaN substrate.
 11. The light source of claim 10, further comprising atleast one cavity spacer layer between said upper mirror and said activeregion.
 12. The light source of claim 11, wherein said conductingchannel electrically contacts said cavity spacer layer.
 13. The lightsource of claim 11, wherein said conducting channel extends into saidcavity spacer layer.
 14. The light source of claim 10, wherein saidconducting channel extends into said active region.
 15. The light sourceof claim 11, wherein said common epitaxial layer comprises a distributedBragg reflector (DBR) layer.
 16. The light source of claim 11, whereinsaid lower mirror includes an acceptor doped region.
 17. The lightsource of claim 10, wherein said light source comprises avertical-cavity surface-emitting laser (VCSEL).
 18. The light source ofclaim 10, wherein said second impurity region is included in at leastpart of said active region.